Esters are most commonly prepared by the reaction of an acid and an alcohol accompanied by the elimination of water. Esters may also be formed by reaction of an alcohol with various other reactants including acid anhydrides, acid chlorides, amides, nitriles, aldehydes, and ketones. Mixtures of acids and/or alcohols may also be used as starting materials. The same may apply for the other reactants.
The reaction conditions under which esterification is effected, can be varied considerably. The reaction proceeds very slowly at room temperature, but quite rapidly at elevated temperatures. Typically one of the reactants is used in stoichiometric excess in order to drive the reaction. The other reactant is then called the limiting reagent. About 99% of the limiting reagent, e.g., acids, anhydrides, alcohols or polyols, can be converted to an ester within a few hours. Limiting reagents are typically reagents which are not present in stoichiometric excess, e.g., limiting reagents used to make plasticizers include diacids and phthalic anhydride and those used to make polyol esters are polyols.
Because the esterification of an alcohol and an organic acid or anhydride is a reversible reaction, the esterification reaction normally does not go to completion. However, conversions of over 99% can be achieved by removing at least one of the esterification products, typically water. If one of the products is boiling at a lower temperature than the other one and than the reagents, this removal is typically achieved by distillation. A variety of distillation techniques are known in the art to remove the produced water from the reaction zone. One method of water removal includes carrying out the reaction in a liquid medium which may form an azeotrope having a boiling point that is lower than that of either or each component of the reaction. If the reagents and the resulting ester have boiling points above 100° C. at atmospheric pressure, then the reaction temperature can be adjusted such that no liquid medium capable of forming an azeotrope with water is required. Additionally, an entrainer may be used to aid in the distillation of the water from the reaction mixture. Inert materials such as cyclohexane, hexane, benzene, toluene, or xylene may be used as an entrainer in the production of phthalate esters. In addition, the reactant having the lower boiling point may also be employed as the entrainer. In this latter case, the reactant used as the entrainer is typically charged into the reaction mixture in excess over the stoichiometric quantities required for the reaction. Esterification processes, including those employing water removal, may be conducted in a batch or continuous mode of operation. Various esterification processes are disclosed in Volume 9 of the Kirk-Othmer Encyclopaedia of Chemical Technology, Fourth Edition (1994), pp. 762-768, the entirety of which is hereby incorporated by reference.
A conventional batch esterification procedure includes charging all of the reactants into the reactor at the beginning of the reaction cycle. In catalytic esterification processes, the catalyst is typically added to the reaction mixture after the batch reaches a target temperature. The reaction mixture may then be heated further. The temperature of the reaction mixture rises until the boiling point of the reaction mixture is achieved, at which point the entrainer, if used, and water by-product boil out of the reaction mixture. Typically, the overhead vapours are condensed, the water separated from the entrainer, and the entrainer recycled to the reactor vessel. The reaction temperature, and therefore the rate of reaction, is limited by the boiling point of the reaction mixture. When the reactant with the lower boiling point is also used as the entrainer, its concentration is gradually reduced as the reaction proceeds. Also the concentrations of the reactants decrease during the reaction, which negatively affects the reaction rate. Thus the reaction temperature, and, therefore, the rate constant for the reaction, increases as the reaction proceeds, irrespective whether an entrainer is used or not, particularly if heat input is continued during the course of the reaction.
One conventional process for forming plasticizer esters is disclosed in UK Patent 1,426,057 (Imperial Chemical Industries Limited), wherein plasticizer esters are prepared from phthalic anhydride and a C4 to C14 alkanol or mixture of such alkanols. For example, a mixture of phthalic anhydride and one or more of these alkanols may be heated gradually up to 180 to 260° C. in the presence of a titanium catalyst (e.g., titanium isopropoxide). When the temperature reaches 180 to 260° C., the esterification is substantially complete although the residual acidity is about 0.3 to 0.05 mg KOH/gram. Aqueous sodium carbonate solution is then slowly added to the ester product to provide 1 to 12 times the stoichiometric amount of alkali. When the temperature has fallen to between 150 and 200° C., water or a dilute aqueous alkali solution is admitted and the excess alkanol is removed. By this treatment, the titanium catalyst is converted to titanium oxide and precipitated, and, thereafter, may be filtered off with excess sodium carbonate and the residual acidity is reduced to less than 0.05 mg KOH/gram.
Conventional esterification processes may be accomplished in two reaction steps. The first reaction step generally occurs in the absence of an esterification catalyst, while the second reaction step may include the use of an esterification catalyst. In U.S. Pat. No. 5,349,075 to Van den Berg et al. a two step esterification process with a first uncatalyzed esterification reaction step conducted at a temperature of at least 200° C., i.e. conditions whereby the more volatile reactant is in the gaseous phase while the less volatile reactant is in the liquid phase, followed by a catalyzed second esterification reaction step at a temperature below 100° C. is proposed. The process employs a solid acid catalyst in the second reaction step.
In the commercial production of plasticizer esters, e.g. phthalates, adipates, and trimellitates, conversions of greater than 99% are desired. For polyol esters, e.g. esters made from aliphatic acids and trimethylolpropane (i.e., the limiting reagent), the commercially desirable conversions are at greater than 98%. Typical polyol ester product applications require conversions of about 98.5% of the original number of hydroxyl groups in the poly alcohol, though applications for products with partial conversion of the hydroxyl groups, such as about 35% unconverted hydroxyl groups, are also known.
Most esterification processes are capable of converting about 99% of the limiting reagent, such as acids, anhydrides or polyols, to an ester within a few hours of reaction time; however, after about 90% of the limiting reagent is converted, the rate of reaction tends to slow down substantially. It may take half as long again to convert the remaining 4-5% of limiting reagent as it took to convert the initial 95% thereof.
The chemical industry is continuously seeking to decrease the reaction time or the batch cycle time of esterification, as well as the quality of the resultant esters. It would therefore be desirable to develop a process which increases the overall rate of reaction especially during the early part of the reaction or of the batch cycle.
The esterification of dibasic acids or acid anhydrides proceeds by the first esterification of one acid group or partial esterification of the anhydride in both instances resulting in the formation of a mono-ester. In this first reaction a catalyst may not be required. Subsequently the esterification is completed to form the di-ester, generally in the presence of a catalyst. The catalytic esterification is typically performed by mixing the dibasic acid or anhydride and the alcohol, and raising the temperature to a certain level at which the catalyst is added. Since at least the formation of the mono-ester is a substantially instantaneous reaction when an anhydride is involved, and otherwise still a faster reaction than the second step, a significant amount of esterification therefore may already occur before the addition of catalyst, whose presence is desired or even required to drive the reaction to completion in a commercially acceptable time.
The esterification reaction produces water and since the titanium or tin esterification catalysts that are typically used are water sensitive, it is necessary to minimize contact between the water produced in the reaction and the catalyst. Accordingly it has been practice to distil off as much as possible of any water of reaction during the initial phases of the reaction, prior to addition of the catalyst. Thus in a typical reaction such as that described in U.S. Pat. No. 6,355,817 B1, a dialkyl phthalate was produced by reacting phthalic anhydride and an alcohol under reduced pressure to maintain a constant boil up of the reaction mixture. However this process requires a relatively long cycle time and also involves boiling off and recycle of reactants, particularly the alcohol. Generally the alcohol is recycled by reflux, which is typically subcooled and can therefore lower the temperature of the reaction mixture and further prolong the reaction cycle time, in particular when heat input capabilities are limited, which is usually the case in an industrial setting.
There remains therefore a need to improve the production of such reactions, to reduce batch cycle times, to improve catalyst use and to generally improve the overall efficiency of the reaction including optimisation of the use of the heat supplied to the reaction. We have now found that the efficiency of the reaction may be significantly improved by the use of a particular temperature and pressure profile during the reaction, and particularly during the initial stages of the reaction.